FWA (Fixed Wireless Access) system is well known as a wireless transmission system employing the above transmission technique. The FWA system is generally composed of two opposing wireless transmission devices and capable of providing a communication service between two distant points (e.g., between a subscriber located at an isolated island and a subscriber located at a mountainous area) without laying cables and the like.
To each wireless transmission device, fixed rate data signals (hereinafter may be referred to as fixed rate signals) as the one specified in ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G.703, for example, are input in parallel, and a variable rate data signal (hereinafter may be referred to as a variable rate signal) as in Ethernet (registered trademark) is also input. Each wireless transmission device multiplexes a variable rate signal and a plurality of fixed rate signals, and transmits the multiplexed signal to an opposing device through one wireless transmission path.
Incidentally, in recent years, the adaptive modulation scheme is often employed by wireless transmission devices. This is because that the adaptive modulation scheme enables both maximization of transmission capacity and securing of minimum transmission capacity according to a state of a wireless transmission path.
Moreover, in many cases, the FDD (Frequency Division Duplex) scheme is used together by wireless transmission devices. In the FDD scheme, different frequencies are used for an uplink wireless transmission path and a downlink wireless transmission path, and it is common that wireless transmission path characteristics are different in the uplink wireless transmission path and the downlink wireless transmission path.
For this reason, a wireless transmission device employing both of the adaptive modulation scheme and the FDD scheme may select different modulation schemes for an uplink wireless transmission path and a downlink wireless transmission path. In such a case, there has been a problem that, in a symmetric transmission path, symmetric property between the uplink wireless transmission path and the downlink wireless transmission path cannot be retained, and in an asymmetric transmission path, a desired transmission ratio cannot be maintained.
As a technique to address this problem, Patent Literature 1, for example, discloses a method capable of using the same modulation scheme in the opposing wireless transmission devices at all times. To be more specific, in Patent Literature 1, when modulation scheme differs between wireless transmission devices due to a difference in characteristics of an uplink wireless transmission path and a downlink wireless transmission path, control is performed such that the modulation scheme is changed to a direction with a lower multiple value.